1. Field of the Invention
This invention relates to a ferroelectric smectic liquid crystal composition and a liquid crystal display element employing this ferroelectric liquid crystal composition. More specifically, it relates to a ferroelectric liquid crystal composition having a good memory characteristic, and to a liquid crystal display element containing the composition.
2. Description of the Related Art
A ferroelectric liquid crystal exhibits a quick electrooptic response and memory effect which are the most significant features of a material for display. There is expected, by utilizing those features, a display having both large display capacity and high display density which had never been produced in a conventional twisted nematic mode display. In order to drive a liquid crystal display element in a multiplexing mode, a memory function of the ferroelectric liquid crystal is indispensable. At present, generally known ferroelectric liquid crystal materials necessitate a cell having a cell gap of 2 .mu.m or less in order to realize a memory function. A least thickness of a cell gap for a practical twisted nematic mode display cell isapproximately 5 .mu.m. Considering a possible short-circuit between eectrodes in a cell having a too small cell gap and a poor yield of the cell due to unevenness of the gap, it has been believed that many difficulties lie before commercial production of the cell having a small cell gap.
As a contrast ratio increases with an increase in cell gap in a guest-host mode display cell employing a ferroelectric liquid crystal, it has been desired, considering the above-mentioned difficulties in the commercial production of the cell having a small gap, to obtain a ferroelectric liquid crystal material which is capable of developing a memory effect even in a cell having a large gap.
It is described in Japanese patent application laid-open No. 159118/1987 that development of memory effect in a ferroelectric liquid crystal display element depends on a tilt angle of the liquid crystal material and that a good memory effect was available when tilt angle was 30 degrees or more. However, the present inventors have experienced that a good memory effect was not necessarily developed even though the tilt angle of the liquid crystal was specified; accordingly, research has been taken place by the inventors longing for a ferroelectric liquid crystal material which is capable of exhibiting a good memory effect in a cell having as large a gap as those of practical twisted nematic mode cells.